volume control level bars gone

This is from I guy I know at a racing forum, could someone give some pointers. thanks

I've got a new HP pavilion running windows xp.Everything works great considering my high level of computer illiteracy.

Except one thing:

I have a volume control button on my keyboard as well as a little icon on my desktop to control the volume.When I first set-up my computer,pressing either of these volume controls would show little vertical green volume indicator 'bars' to show how loud the volume was.Ok,after about the first month of use,these little bars just vanished!In other words,I press the Volume control and the volume still works properly....just the bars to indicate the sound are gone!I know it's a little thing but how do I get the bars to show again?
I used the system restore and set my system back a spell...and the bars are there.After about a week they vanish again!I tried everything I know of...the sound scheme thing in the control panel,etc....anybody got any ideas??